The ink jet recording system is a recording method capable of printing at a high speed in low noise, and recently has become rapidly widespread.
The ink jet recording system comprises discharging a liquid ink having high fluidity from a narrow nozzle to recording paper and is divided into a continuous discharging system and an on-demand discharging system. The continuous discharging system includes an electrostatic type (Sweet type, Hertz type), and the on-demand discharging system includes a piezoelectric type, a thermal ink jet type and an electrostatically accelerating type, as well known in the art.
Of the ink jet recording system of on-demand type utilizing static electricity, a method called an electrostatically accelerating type ink jet or slit jet is known as described in Susumu Ichinose and Yuuji Ooba, Denshi Tsusin Gakkai Ronbunshi, Vol. J66-C, No. 1, page 47 (1983) and Tadayoshi Oono and Mamoru Mizuguchi, Gazo Denshi Gakkaishi, Vol. 10, No. 3, page 157 (1981). Such an ink jet recording method is also described more specifically, for example, in JP-A-56-170, JP-A-56-4467 and JP-A-57-151374 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
According to the method, ink is supplied from an ink tank to a slit-shaped ink chamber having many electrodes arranged in inner surfaces of slit-shaped ink retaining part and when a high voltage is selectively applied to the electrodes, the ink neighboring to the electrode is discharged on a recording paper closely positioned against the slits, thereby conducting recording.
A method which does not use such a slit-shaped recording head is also known. In JP-A-61-211048, there is described a method in which pores of a film-like ink retainer having plural pores are filled with ink and the ink in the pores is transferred to a recording paper by applying selectively a voltage to the ink using a multi-needle electrode.
It is believed that the theory of the ink flight is that the high voltage applied to the electrode arranged pours electric charge into the ink adjacent to the electrode and the charged ink neighboring to the electrode is ejected upon the electrostatic power generated. Therefore, the ink is normally not charged but only when the voltage is applied, the ink neighboring to the electrode is charged to get power for the ejection.
The ink employed for these methods is that having electric resistance of from about 10.sup.6 to about 10.sup.8 .OMEGA.cm. Since water has low electric resistance, an oily solvent containing a coloring agent such as a dye dispersed therein with a dispersing aide such as a surface active agent to control the electric resistance thereof is ordinarily employed.
Further, various proposals for controlling the properties of oil-based ink have been made. For example, there are a method of controlling viscosity and specific resistance of the oil-based ink as described in JP-B-52-13127, a method of controlling dielectric constant of a dispersion medium used in the ink and specific resistance of the ink as described in JP-A-53-29808, and a method of varying the dispersion medium for the oil-based ink or a method of incorporating a specific compound into the ink composition as described in JP-A-3-79677, JP-A-3-64377, JP-A-4-202386 and JP-A-7-109431.
However, the oil-based ink using these known techniques are still insufficient in view of their properties, for example, preservation stability of the oil-based ink, reproducibility of recording image at the time of repeated use of the ink, blur of the ink on an image receiving material, clogging of the ink in a nozzle or an ink delivery channel, and stability on ink ejection. More improvements in these properties have been therefore desired.
Another ink jet technique of an electrostatic type is described in WO 93/11866. This method comprises, after supplying ink containing charged particles or particles which are chargeable under an electric field dispersed therein to an ink ejection apparatus, a series of steps composed of (1) forming ink meniscus at the tip of an ejection electrode for ejecting the ink, (2) increasing particle density in the ink meniscus upon electrophoretic concentration of particles, and (3) ejecting agglomerations of the particles away from the ejection electrode by forming an electric field between the ejection electrode and a counter electrode bearing a recording medium.
This method which does not use a nozzle structure contrary to conventional methods has many advantages in that ink containing dispersed particles such as pigment can be ejected as minute droplets having a particle size of several .mu.m, in that the droplet ejected can have a high concentration of particles, and in that a dot size of image can be varied by changing a size of the droplet by means of controlling an ejection signal.
Accordingly, images composed of pigment having good light-fastness and water-resistance can be formed and clear images of high resolution and high density including continuous gradation dot images can be obtained.
The oil-based ink to be used include those comprising an electrically insulating liquid having an electric resistance of 10.sup.9 .OMEGA.cm or more containing insoluble and chargeable particles and charging agents therein as described in WO 95/1404 and WO 96/10058.
Oil-based ink in which an amount of charge of particles or an average particle size of particles is defined in the specific range as described in JP-A-9-193389 and JP-A-8-291267, and oil-based ink in which thermal properties of solid material in the ink composition are specified as described in JP-A-9-137094 are also proposed.
However, when ink jet recording was conducted using such known oil-based ink, due to unstable ink ejection or insufficient concentration of pigment particles in the ink, disappearance or blur of image formed and poor image density, particularly in a solid image area were observed. Further, in case of using ink preserved for some time, an ejection condition such as a voltage applied was varied from the use of fresh ink and a rate of concentration and ejection remarkably changed. That is, a problem that quality of image obtained varied depending on the preservation condition of the ink arised.
With development in business appliances and progress in office automation in recent years, a plate-making system wherein an image is formed on a lithographic printing plate precursor comprising a water-resistant support having provided thereon an image-receiving layer in a various manner to prepare an offset lithographic printing plate has become widespread in the field of small-scale commercial printing. The ink jet recording method can be employed for the image formation in such a plate-making system. However, when printing was actually conducted using a printing plate having clear images formed using conventional oil-based ink, only several hundred sheets of prints having clear images could be obtained. This is clearly not satisfactory to printing durability. Specifically, there is a problem that the strength of ink particles fixed on the printing plate is insufficient for offset printing.